Sudden Infant Death Syndrome (SIDS) is one of the leading causes of death in the first year of life. Studies in the last decade have clarified the definition and epidemilogy of this disorder. However, to date no mechanism has been found to account for the sudden death of apparently normal infants, mostly during sleep, and peaking at about 3 months of age. Of the major hypothesis advanced to explain SIDS, the potentially most significant relates to the role of the autonomic nervous control of the cardiorespiratory system during sleep. We are currently investigating the hypothesis that SIDS is a disorder of the autonomic nervous system. The Biochemical markers of autonomic nervous function which we are measuring are the neurotransmitters and the enzymes of neurotransmitter metabolism in heart, brain stem and other tissues. Demonstration of biochemical correlates of SIDS can serve as a basis for the development of clinical chemical tests for the identification of potential SIDS victims in life.